This invention relates to optical systems and in particular to optical imaging systems which are capable of multi-spectral imagery.
According to the present invention there is provided a two-stage multi-spectral imaging system comprising a reflective objective and a reflective relay aligned on a common optical axis, the objective being arranged to form an intermediate image and the relay being arranged to deliver image-forming radiation emanating from the intermediate image to a focal plane, wherein the objective is formed by a large concave primary mirror which is apertured on-axis and a small secondary mirror generally disposed in the form of a xe2x80x9cCassegrainxe2x80x9d objective, the relay is formed by a pair of confronting mirrors each being apertured on-axis to enable passage of radiation into and out of the relay, and the relay being positioned to provide space between the relay and the focal plane to accommodate waveband selective splitting optics if so desired.
Preferably a field stop is located at the apertured centre of the primary objective mirror.
Preferably a field lens which is transmissive over all desired spectral wavebands is located adjacent the intermediate image at the apertured centre of the primary objective mirror to provide pupil imaging.
Preferably waveband-selective splitting optics are provided at a pupil in said space, said optics comprising at least a dichroic mirror arranged to transmit longer wavelengths and to reflect shorter wavelengths. Said optics may further comprise a beam splitter which may be neutral or dichroic to handle the reflected shorter wavelengths.
Conveniently the dichroic mirror transmits mid and/or far infrared wavebands.
Within what is included herein as being xe2x80x9cCassegrainxe2x80x9d the secondary mirror of the objective may be generally planar or it may be convex. Furthermore xe2x80x9cmirrorsxe2x80x9d which are said to be apertured may be provided either by a substrate with a hole or opening or by a substrate which is transmissive to radiation in the relevant wavebands at the location of the aperture.
The relay mirrors are preferably concave. The various mirrors of the system together with the housing for the system are preferably made of the same material such as aluminium so that the system is inherently athermal. Also where the housing is sealed and radiation ingress is via a window it may be necessary to compensate the system for chromatic and thermal effects if the window is optically powered (as would be the case if the window were dome shaped) in which case it is preferred to make one of the mirrors a Mangin mirror, for example a relay mirror.